1. Field of the Invention
This invention relates to a transformation coding device which applies a highly efficient coding to a digital image signal.
2. Description of the Prior Art
FIG. 1 is block connection diagram showing a a conventional transformation coding device described in, for example, the paper entitled "A proposal of a coding control method in a MC-DCT coding system" by Kato et al., at the National Conference on Information and System Dept. (1-204) of the Institute of Electronics, Information and Communication Engineers, 1987. In FIG. 1, reference numeral 11 is a subtractor which performs subtraction between an input signal 101 as a digital image input signal series and a prediction signal 102 between movement compensating frames, 1 is an orthogonal transformation unit which performs orthogonal transformation for a difference signal 103 between frames, 2b is a quantization unit which applies a threshold processing to a transformation coefficient 104 obtained by the orthogonal transformation in accordance with an amount of data storage in a buffer 112 and then outputs a quantization index 106, 3b is a quantization decoding unit which applies a quantization decoding to the quantization index 106 and outputs a quantization transformation coefficient 107, 4 is an inverse orthogonal transformation unit which applies inverse orthogonal transformation to the quantization transformation coefficient 107 and produces a difference signal between decoding frames 108, 12 is an adder, 5 is a frame memory which produces the prediction signal 102 between movement compensating frames, 6 is a movement compensating unit, 7 is a variable length coding unit, 111 is coded data, 8 is a buffer for transmission, and 113 is transmitted data.
Next, the operation will be described. The difference between the input signal 101 and the prediction signal 102 between movement compensating frames is obtained using the subtractor 11 to produce the difference signal 103 between frames from which redundant components are removed. In the orthogonal transformation unit 1, the difference signal 103 between frames is transformed into a spatial frequency region using orthogonal transformation to produce the transformation coefficient 104. In the quantization unit 2b the transformation coefficient 104 is subjected to the following threshold processing based on an amount of buffer storage data described later.
An amount of buffer storage: Large .fwdarw.Threshold value Th: Large PA0 An amount of buffer storage: Small .fwdarw.Threshold value Th: Small PA0 Ci.gtoreq.Th.fwdarw.Ci: Insignificant coefficient PA0 Ci&gt;Th.fwdarw.Ci: Significant coefficient
Where Ci: transformation coefficient.
The transformation coefficient 104 which is a significant coefficient as the result of the threshold processing is quantized in the quantization unit 2b and outputted as a corresponding index 106. On the other hand, the transformation coefficient 104 which is an insignificant coefficient is output as the quantization index 106 corresponding to zero. The quantization index 106 is subjected to a variable length coding together with a movement vector 105 described later and outputted as a coded data 111 from the variable length coding unit 7. On the other hand, the quantization index 106 is simultaneously transformed into the transformation coefficient 107 as the result of quantization decoding in the quantization decoding unit 3b. In the inverse orthogonal transformation unit 4, the transformation coefficient 107 is transformed into the difference signal 108 between decoding frames. Subsequently, in the adder 12, the difference signal 108 between decoding frames and the above-mentioned prediction signal 102 between movement compensating frames are added to produce a decoded signal 109. The decoded signal 109 is temporarily stored in the frame memory 5 to produce the prediction signal 102 between movement compensating frames as the result of the movement compensation. Also, in the movement compensating unit 6, after the input signal 101 is divided into a plurality of blocks, displacement between the most similar portions, defined by the sum of the absolute value of the difference between two blocks or the sum of squares of the difference etc. are calculated using a preceding frame demodulated signal 110 located at the spatially same position or in the vicinity for each block, portions which provide the minimum displacement are detected from image signal 110 of the preceding frame, and the minimum amount of spatial displacement is outputted as the movement vector 105. The above-mentioned coded data 111 is temporarily stored in the buffer 8 for transmission, outputted as transmitted data 113 with a fixed bit rate, and at the same time, an amount of buffer data storage 112 is outputted as a feedback signal in order to prevent buffer overflow. Since the conventional transformation coding device is constituted as described above and a selection of quantization characteristics is performed by using only an amount of buffer data storage for transmission, there is a defect that it is difficult to carry out adaptive quantization in accordance with statistical properties of an input signal such as power and frequency characteristics, and it is impossible to compress a signal efficiently.